Eddy current losses tend to exceed the I.sup.2 R losses in high speed, high power switched reluctance machines as will be utilized for internal jet-engine starter-generators. These machines are characterized by a low number of turns per phase made from rectangular, hollow conductors with outside dimensions of typically 0.090 inch by 0.125 inch or larger. These conductors are exposed to high magnetic fields which change amplitude very rapidly at a rate of typically more than 1000 Hz. the electrical losses in the conductors, generally called I.sup.2 R losses, are much larger than the losses calculated by multiplying the square of the rms value of the winding current times the DC resistance of the conductor. These additional losses are due to eddy currents induced by the rapidly changing magnetic field in the solid walls of the conductor. Normally these eddy current losses are between 10 to 800% of the DCI.sup.2 R losses. However, in high speed, high power switched reluctance machines, the additional losses due to the eddy currents can be a multiple of the DCI.sup.2 R losses. Applying the same machine type to an internal starter/generator, the power rating and the fundamental frequency can readily be higher than for an external starter generator, potentially resulting in very high eddy current losses.
High eddy current losses introduce a number of challenges. For example, the cooling required by the high eddy current losses challenges the conventional cooling system for which standard engine lubrication oil is used. Also, the adverse impact on efficiency of the system becomes rather significant with high eddy current losses.
It would be desirable, then, to be able to reduce the eddy current losses in internal starter/generator arrangements.